d-transition elements (metals) Transition elements
are elements that can form bonds with the electrons
from the outer two shells. They are characterized by
having an incompletely filled d subshell. There are
three series of transition elements, corresponding to fill-
ing of the 3d, 4 d,and 5dsubshells. They are all metals,
and most are hard, strong, and lustrous, have high
melting and boiling points, and are good conductors of
heat and electricity.
Transition elements that are common in minerals,
or occur in significant amounts, are titanium,
chromium, manganese, iron, cobalt, nickel, copper,
molybdenum, silver, tungsten, gold, and platinum.
d-Transition elements are B Group elements except
for IIB in the periodic table. In the newer labeling
scheme, they are groups 3 through 11.
dual-action drug A compound that combines two
desired different pharmacological actions in a similarly
efficacious dose.
dual substituent-parameter equation In a general
sense, this is any equation that expresses substituent
effects in terms of two parameters. However, in prac-
tice, the term is used more specifically for an equation
for summarizing the effects of meta- or para-sub-
stituents (i = m or p) X on chemical reactivity, spectro-
scopic properties, etc., of a probe site Y in a benzene or
other aromatic system.
Pi= ρIiσI+ ρRiσRP is the magnitude of the property Y for substituent X,
expressed relative to the property for XH; σIand σR
are inductive or polar and resonance substituent con-
stants, respectively, there being various scales for σR; ρI
and ρRare the corresponding regression coefficients.
See alsoEXTENDEDHAMMETT EQUATION.
ductile The ability of a material to be drawn out or
hammered in thin pieces or fashioned into a new form.
Dumas method A way to determine the MOLECULAR
WEIGHTs of volatile liquids.dynamic equilibrium An EQUILIBRIUMin which pro-
cesses occur continuously or the actions oppose each
other so that no net change occurs.dynein A molecular motor, a complex believed to be
made of 12 distinct protein parts, that performs basic
transportation tasks critical to the cell. Converts chemi-
cal energy stored in an ATP (adenosine triphosphate)
molecule into mechanical energy that moves material
through the cell along slender filaments called micro-
tubules. One of the most important functions occurs
during cell division, when it helps move chromosomes
into proper position. It also plays a part in the move-
ment of eukaryotic flagella and cilia.
Molecular motors play a critical role in a host of
cell functions such as membrane trafficking and cell
movement during interphase, and for cell asymmetry
development. During cell division, they are responsible
for establishing the mitotic or meiotic spindle, as well
as segregating chromosomes and dividing the cell at
cytokinesis. It is the last part of the mitotic cycle, dur-
ing which the two daughter cells separate. Motors
either move along actin tracks (members of the myosin
superfamily) or microtubules (the dynein and kinesin
superfamilies). Based on the Greek dunamis,meaning
power.dyotropic rearrangement An uncatalyzed process in
which two SIGMA(σ) BONDs simultaneously migrate
intramolecularly, for example,86 d-transition elements